For convenient and efficient testing of clinical samples of biological fluids, small precise quantities of stable diagnostic reagents are needed. These reagents must be efficiently and economically prepared in large quantities without sacrificing precise delivery of the reagents. Further, the reagents should be delivered to the user in a stabilized form so as to prevent wastage of expensive reagents. The form in which the reagents are provided must be suitable for use in simple and rapid testing without the intervention of highly skilled technicians. One form which can meet these needs is a tablet containing all the reagents necessary to conduct a given diagnostic assay.
A tablet of this type needs to be stable, easily prepared in a highly reproducible manner, and to dissolve rapidly upon mixing with an appropriate sample. There must be good tablet-to-tablet reproducibility, which in turn means that the dry powder blend from which the tablets are made must be homogeneous. The reagents must withstand the conditions used to prepare the powder blend and then the tablets, and the resulting tablet must be easily dissolved in aqueous solutions.
Preferred tablets for use in diagnostic applications are very small, preferably less than 50 mg and more preferably less than 10 mg. The need for such small tablets compounds the normally difficult problems of producing tablets useful as carriers of diagnostic reagents. The problem of inhomogeneity of the dry powder blend used to form the small tablets is particularly severe as even minor inhomogeneities will have large adverse effect on the tablet-to-tablet reproducibility. This is so because of the relatively small number of dry powder particles used to form each tablet. In order to obtain the necessary homogeneity, techniques such as the S-1 spray freeze process are required (see, for example, U.S. Pat. No. 3,932,943, issued Jan. 20, 1976, and U.S. Pat. No. 3,721,725, issued Mar. 20, 1973, both to Briggs et al.).
Use of tablets prepared by the S-1 spray freeze process, which contain all the ingredients necessary to perform a diagnostic assay, are limited to those diagnostic assays in which all the ingredients are compatible in a single aqueous solution. That means, for example, that an assay requiring both an enzyme and its substrate cannot be provided as a single tablet formed from a blend made by the S-1 process. This is so because when the solution to be sprayed is prepared, the enzyme and substrate react consuming the substrate making it unavailable in the final tablet.
Various methods have been employed in the past to obtain a dry product containing at least two materials which are incompatible with each other in aqueous solution. None of these methods produces materials which are suitable for compressing into the small tablets needed as carriers of diagnostic reagents because these methods ultimately required bulk lyophilization of a frozen mass and it is known that bulk lyophilization produces powders with significant inhomogeneities. Exemplary processes are those disclosed by Damaskus, U.S. Pat. No. 3,269,905, issued Aug. 30, 1966; Barclay, U.S. Pat. No. 3,616,543, issued Nov. 2, 1971; Price et al., U.S. Pat. No. 3,862,302, issued Jan. 21, 1975; Krupey, U.S. Pat. No. 4,295,280, issued Oct. 20, 1981; and Hurwitz et al. U.S. Pat. No. 4,351,158, issued Sept. 28, 1982. Damaskus describes a method of freezing successive layers of incompatible materials in a container followed by bulk lyophilization. Barclay describes a method where solutions of incompatible materials are sequentially charged into a container with freezing of the charge and rotation of the container between charges so that the separate charges do not touch, again followed by bulk lyophilization. Price et al. describe a method in which solutions of incompatible materials are separately formed into frozen beads, the frozen beads are lyophilized separately. The only method disclosed for mixing the dried beads is counting of the number of beads added to a vial. There is no method shown for combining the beads prior to lyophilization nor is there any disclosure aimed at insuring complete mixing of the two populations of beads so that concentration gradients are not formed in the resulting powder. Krupey describes a method in which the solutions of the incompatible materials are first cooled, then mixed together and immediately charged into a container which had been cooled substantially below the freezing temperature of the solutions. The frozen charge is then subjected to bulk lyophilization. This method is further limited in that the amount of material processed is limited by the volume which can be charged into the freezing container rapidly. If large volumes are to be processed, the time required to charge the container will be too long allowing the incompatible materials to react with each other. Hurwitz et al. describe a method of charging two solutions of incompatible materials into separate portions of a container where they are immediately frozen so as not to come into contact with each other. These charges are then bulk lyophilized. None of the methods disclosed offers a satisfactory solution to the problem of preparing homogeneous dry powders suitable for forming into compressed tablets useful as carriers of diagnostic reagents.
Thus, there is a need for an improved spray-freeze process which allows production of tablets containing incompatible ingredients.